1. Field of the Invention
The invention relates to a method and apparatus for depositing thin layers of insulating or slightly conductive materials by reactive spraying through a high-frequency inductive plasma.
The invention relates to a method wherein deposition is produced by spraying the conductive component of the material to be deposited, which comprises the means for applying a high-frequency electromagnetic field, owing to the presence of a gas comprising a reactive gas and an inert gas which can be ionized by the electromagnetic field.
The invention relates to an apparatus comprising a vessel connected to a vacuum generation system containing means which are adapted to produce a high-frequency electromagnetic field and which comprise a material which can be sprayed by the field, at least one surface serving as a substrate for thin layer deposition, and means for injecting a gas comprising a reactive gas and an inert gas ionizable by the field into the vessel near the means for applying the electromagnetic field.
2. Description of the Prior Art
As is well known, various methods such as evaporation, cathode spraying by high-frequency inductive plasma have been used for producing thin layers of materials comprising a number of components, more particularly insulating or slightly conductive materials such as oxides, nitrides or carbides.
In the evaporation method, the insulating components have a low vapour pressure and a high melting point and can be evaporated only by using sources of intense heat such as electron bombardment or an electric arc, in order to form the layers at a sufficient speed. Another disadvantage is that the component to be deposited may also become dissociated owing to the high temperature and the negative pressure.
Another disadvantage of depositing insulating materials by cathode spraying is that the layers are formed very slowly, since a space charge occurs on the cathode and prevents any d.c. spraying occurring; even if radio-frequency is used, the spray cannot be maintained except by greatly reduucing the rate of spraying.
In the known method of spraying by a high-frequency inductive plasma, the material to be sprayed has to be in coil form. The need to convert the insulating material into a coiled wire is difficult and frequently impossible to overcome in the case of fragile insulating materials having low ductility. Furthermore, the coil must be conductive at radio-frequency, i.e. a coil made of conductive material must be coated with the insulating material to be sprayed.
This increases the difficulty of manufacture, with a considerable increase in cost. Furthermore, in the case of a coil having an insulating surface and in the substantially similar case of cathode spraying of insulating materials, the rate of layer formation is limited owing to the considerable decrease in the rate of spraying.
Finally, the same method of spraying by high-frequency inductive plasma has been used, i.e. a conductive material, e.g. a metal, is sprayed and reacted with a suitable ionizing gas for producing the desired insulating layer. The gas may be both ionizable and reactive, or may be a mixture of a reactive gas and an easily ionizable carrier gas. In this method, particles are sprayed as a result of impacts between the ionized gas and the coil material and react with the reactive gas to form a compound of the material, e.g. an oxide, nitride, carbide, sulphide, etc.
However, in such a method the surface layers of the coil are themselves in direct contact with the reactive gas and react with the gas and form the desired insulating compound, thus producing a surface insulating layer on the turns of the coil and greatly limiting the spraying speed.
Furthermore space charges occur between the turns and produce small electric arcs which abruptly destroy the insulating layer formed. The arcs abruptly evaporate and spray large drops of coil material which, before they can combine, are deposited on the substrate and impair the quality of the deposit.
The invention relates to a method which comprises the reactive spraying of a mixed gas by a high-frequency inductive plasma so that insulating materials may be deposited at a rate substantially comparable with the rate for conductive materials to obtain high-quality uniform deposits.